Barclay C J, Lichtwark G A
School of Physiotherapy and Exercise Science, Griffith University, Gold Coast, PMB50 Gold Coast Mail Centre, Gold Coast, Qld. 9726, Australia.
J Biomech. 2007;40(14):3121-9. doi: 10.1016/j.jbiomech.2007.03.024. Epub 2007 May 11.
The dynamic properties of relaxing skeletal muscle have not been well characterised but are important for understanding muscle function during terrestrial locomotion, during which a considerable fraction of muscle work output can be produced during relaxation. The purpose of this study was to characterise the force-velocity properties of mouse skeletal muscle during relaxation. Experiments were performed in vitro (21 degrees C) using bundles of fibres from mouse soleus and EDL muscles. Isovelocity shortening was applied to muscles during relaxation following short tetanic contractions. Using data from different contractions with different shortening velocities, curves relating force output to shortening velocity were constructed at intervals during relaxation. The velocity component included contributions from shortening of both series elastic component (SEC) and contractile component (CC) because force output was not constant. Early in relaxation force-velocity relationships were linear but became progressively more curved as relaxation progressed. Force-velocity curves late in relaxation had the same curvature as those for the CC in fully activated muscles but V(max) was reduced to approximately 50% of the value in fully activated muscles. These results were the same for slow- and fast-twitch muscles and for relaxation following maximal tetani and brief, sub-maximal tetani. The measured series elastic compliance was used to partition shortening velocity between SEC and CC. The curvature of the CC force-velocity relationship was constant during relaxation. The SEC accounted for most of the shortening and work output during relaxation and its power output during relaxation exceeded the maximum CC power output. It is proposed that unloading the CC, without any change in its overall length, accelerated cross-bridge detachment when shortening was applied during relaxation.
放松状态下骨骼肌的动力学特性尚未得到充分表征，但对于理解陆地运动过程中的肌肉功能很重要，在此过程中，相当一部分肌肉功输出可在放松阶段产生。本研究的目的是表征小鼠骨骼肌在放松过程中的力-速度特性。实验在体外（21摄氏度）使用来自小鼠比目鱼肌和趾长伸肌的纤维束进行。在短强直收缩后的放松阶段，对肌肉施加等速缩短。利用不同缩短速度下不同收缩的数据，在放松过程中每隔一段时间构建力输出与缩短速度的关系曲线。速度分量包括串联弹性成分（SEC）和收缩成分（CC）缩短的贡献，因为力输出并非恒定。在放松早期，力-速度关系呈线性，但随着放松进程逐渐变得更弯曲。放松后期的力-速度曲线与完全激活肌肉中CC的曲线具有相同的曲率，但V(max)降低至完全激活肌肉中值的约50%。对于慢肌和快肌以及最大强直收缩和短暂次最大强直收缩后的放松，这些结果都是相同的。所测量的串联弹性顺应性用于在SEC和CC之间划分缩短速度。CC力-速度关系的曲率在放松过程中保持恒定。SEC在放松过程中占缩短和功输出的大部分，其在放松过程中的功率输出超过CC的最大功率输出。有人提出，在放松过程中施加缩短时，在CC总长度不变的情况下卸载CC会加速横桥解离。
